Evolution of residual stress in environmental barrier coatings for ceramic matrix composites

陶瓷基复合材料(CMCs)是一种用于燃气轮机发动机的激动人心的新材料，其中需要一种环境阻挡涂层(EBC)来保护CMCs免受水蒸气的侵蚀。该涂层还可用作热障涂层(TBC)。这种涂层可以延长使用寿命，还可以提供更高的使用温度和更高的燃油效率。值得注意的是，环境阻隔涂层是一种主要的依赖涂层(与TBC不同)，不能在使用中失效，因为如果没有它，CMC组件在恶劣的燃气轮机运行环境中的寿命将非常有限。众所周知，环境屏障涂层寿命的一个主要因素是其残余应力状态，这是由其制造过程中通过热喷涂工艺进行的淬火过程和喷涂后的冷却过程造成的。涂层的应力状态也会随着热漂移而改变，例如热处理和发动机操作。这是由于热膨胀系数(CTE)、热梯度、涂层结晶、相变和烧结等因素造成的。然而，关于它们如何结合在一起改变应力状态的知识很少，如果没有应力状态，目前就不可能预测它们的演变，也不可能对EBC寿命产生影响。该项目的目的是为涂层应力随时间和温度的演变发展新的理解和模型。目标包括对EBC结晶、相变、烧结和热机械载荷对涂层内部应力及其完整性的影响的新观察。该项目将采用新的实验方法来研究高分辨率的应力和应变状态，包括同步X射线衍射、同步加速器和实验室X射线计算机层析成像，以及使用聚焦离子束球磨的微尺度钻孔，通过二维和三维数字图像相关进行分析。这项研究将主要使用热处理涂层的非原位观察，但可能会使用英国国家设施的同步加速器X射线进行原位研究。结果将在模拟中使用有限元方法的建模来解释，该模拟将包括热物理过程，例如衬底和EBC之间的粘结层的蠕变。该项目属于EPSRC材料工程-陶瓷研究领域，在本网站https://epsrc.ukri.org/research/ourportfolio/themes/列出的工程主题中这是一个由劳斯莱斯公司支持的iCASE项目。

Ceramic matrix composites (CMCs) are exciting new materials for use in gas turbine engines, in which an environmental barrier coating (EBC) is needed to protect the CMCs from water vapour attack. The coating also serves as a thermal barrier coating (TBC). The coating acts to increase the operating lifetime and can also offer higher operating temperature with improved fuel efficiency. Significantly, the environmental barrier coating is a prime reliant coating (unlike a TBC), that must not fail in service, since without it the CMC components would have very limited life in the harsh gas turbine engine operating environments. It is known that one major factor governing the life of an environmental barrier coating is its residual stress state, which results from the quenching process of its manufacture via the thermal spray process and the cooling after spraying. The coating stress state also changes with thermal excursions such as heat-treatment and engine operations. This is because of factors that include the mismatch of coefficients of thermal expansion (CTE), thermal gradients, coating crystallization, phase change and sintering, among others. However, there is little knowledge of how these combine to change the stress state, without which it is not currently possible to predict their evolution, nor the effect on the EBC lifetime. The aim of this project is to develop new understanding and models for the evolution of coating stress with time and temperature. The objectives include new observations of the effects of EBC crystallization, phase change, sintering, and thermal mechanical loading on the stresses within the coating and its integrity. The project will apply novel experimental methodologies to investigate the state of stress and strain at high resolution, including synchrotron X-ray diffraction, synchrotron and laboratory X-ray computed tomography, and micro-scale hole-drilling using focused ion beam milling, analysed by 2D and 3D digital image correlation. The study will mostly use ex situ observations of thermally treated coatings, but in situ studies may be achieved using synchrotron X-rays at UK National Facilities. The results will be interpreted using modelling by the finite element method in simulations that will incorporate thermophysical processes such as creep of the bond coat between the substrate and the EBC. This project falls within the EPSRC Materials engineering - ceramics research area' in the Engineering theme listed on this website https://epsrc.ukri.org/research/ourportfolio/themes/ This is an iCASE project, with the support of Rolls-Royce Plc.